From my limited experience with main river fishing...you will usually find the best action in adjoining slack water areas. If there is any current, structure and cover become very important. In significant current, the crappie will hang out on the downstream side of cover in the slack-water eddy caused by the obstruction of flow.

I'm sure that there are large concentrations of crappie in the main river, but it will be difficult to "stay on" the crappie day in and day out. Considering that we are discussing the Mighty Mississippi, there will usually be signifcant fluctuations in water level from week to week. River crappie can be outstanding or leave you skunked. Other than that, I would give it a shot. You don't know 'till you try it.